A fuel cell is a device which, when supplied with a fuel such as hydrogen and with atmospheric oxygen, causes the fuel and oxygen to react electrochemically, producing water and directly generating electricity. Because fuel cells are capable of achieving a high energy conversion efficiency and have an excellent environmental adaptability, they are being developed for a variety of applications, including small-scale local power generation, household power generation, simple power sources for campgrounds and the like, mobile power sources such as for automobiles and small boats, and power sources for satellites and space development.
Such fuel cells, particularly polymer electrolyte fuel cells, are built in the form of modules composed of many unit cells stacked in series. Each unit cell has a membrane electrode assembly which is composed of a solid polymer electrolyte membrane and, disposed on either side thereof, an anode and a cathode. The membrane electrode assembly is in turn sandwiched between a pair of separators.
The anode and cathode are generally each composed of two layers: an electrocatalyst layer positioned on the electrolyte membrane side, and a gas diffusion layer positioned on the separator side. Therefore, the membrane electrode assembly overall is generally composed of five layers.
In recent years, the structure which consists of carbon fiber has started to be used as the substrate for such electrocatalyst layers and gas diffusion layers.
For example, Patent Document 1 discloses a catalyst-loaded carbon nanofiber (sometimes abbreviated below as “CNF”) electrocatalyst layer for fuel cells which is in the form of a nonwoven fabric composed of nitrogen-containing carbon nanofibers and catalyst particles bonded to the nitrogen atoms.
Patent Document 2 discloses a fuel cell catalyst electrode composed of a catalyst supported on an electrically conductive continuous fiber sheet.
Patent Document 3 discloses a gas diffusion layer composed of a woven fabric of carbon fiber filaments having a fiber size of from 0.3 to 1.5 decitex.
Patent Document 4 discloses a carbon alloy catalyst composed of nanofibers having a nanoshell structure. In this carbon alloy catalyst, the carbon structure itself which makes up the nanofibers has a catalytic activity.
The electrocatalyst layer of Patent Document 1 is characterized by including nitrogen atoms in the carbon nanofibers in order to prevent sintering of the supported catalyst particles. Fabricating such nitrogen-containing carbon nanofibers requires low-temperature heat-treating at not above 900° C. At such temperatures, the carbonization that takes place is insufficient, resulting in a poor electrical conductivity. Therefore, when this electrocatalyst layer is to be used as a membrane electrode assembly, it is necessary either to add an electrically conductive material or to apply treatment so as to impart conductivity to the fibers themselves.
Moreover, Patent Document 1 does not mention the use of the electrocatalyst layer as a gas diffusion layer.
In the electrocatalyst layer of Patent Document 2, the catalyst loading in the working examples is high at 1 mg/cm2. Moreover, even compared with a catalyst on a particulate carbon layer, the difference in maximum electromotive force is only on the order of 1.3-fold or more. This is attributable to the fact that, owing to the insufficient flexibility of the conductive fiber nonwoven fabric, adhesion at the membrane-bonding interfaces is poor, thus making it impossible to lower the resistances at the membrane-bonding interfaces of the catalyst-loaded conductive fiber nonwoven fabric surface with the electrolyte membrane and the gas diffusion layer by merely lowering the sheet resistance at the surface of the conductive fiber nonwoven fabric.
Moreover, in Patent Document 2, carbon paper is used in fuel cell fabrication as a support having gas diffusivity. The use of the electrocatalyst layer also as a gas diffusion layer is not mentioned.
The woven fabric composed of carbon fiber filament bundles which makes up the gas diffusion layer in Patent Document 3 is, on a macroscopic level, relatively supple and thus can be subjected to rolling treatment. However, when the woven fabric is examined on a microscopic level, the fiber diameter, at about 6 μm, is large and so the individual fibers are rigid. In addition, because the fibers are in the form of filament bundles, fuzz generation readily occurs, as a result of which the fibers may pierce the electrolyte, which may lead to shorting between the anode and cathode and fuel gas leakage. Hence, it is necessary to carry out surface treatment on the gas diffusion layer.
Moreover, Patent Document 3 does not mention the use of the gas diffusion layer as an electrocatalyst layer.
In the art of Patent Document 4, carbon alloy catalyst (nanoshell structure) particles are formed within nanofibers, thereby preventing coarsening of the particles and enabling fine catalyst particles to be obtained. In this carbon alloy catalyst, much catalyst is present at the interior of the fibers, but this catalyst does not contribute to the redox reactions. Carbon catalyst particles, in and of themselves, have a poor electrical conductivity, and fibers which are formed substantially of catalyst particles alone also have a poor conductivity.
Hence, it is substantially impossible with such nanofibers alone to impart the functions of both a gas diffusion layer and an electrocatalyst.
As noted above, both a gas diffusion layer and an electrocatalyst layer are fundamentally required in the art of Patent Documents 1 to 4.
On the other hand, Patent Document 5 discloses a membrane electrode assembly obtained by depositing a catalyst precursor on a carbon nanofiber nonwoven fabric, then heat-treating the catalyst precursor, which membrane electrode assembly has both a gas diffusing ability and an electrode performance.
However, in the art of Patent Document 5, the catalyst is limited to a carbon catalyst. Hence, as will be described in detail subsequently in the comparative examples, because the carbon nanofibers used are brittle structures lacking in flexibility, the nonwoven fabric structure breaks down under the forces incurred during molding of the membrane electrode assembly, causing gas diffusivity and water drainage by the resulting membrane electrode assembly to decline. These declines are in turn accompanied by a large drop in the power-generating performance when the membrane electrode assembly is used in a fuel cell.